With the increasing traffic demand, cellular operators plan to evaluate the feasibility of using unlicensed spectrum for LTE for Rel-13, i.e. LTE-U. The first stage of LTE-U would aggregate the unlicensed carrier as a Supplemental Downlink (SDL) with a primary carrier in licensed bands.
On these unlicensed bands/channels, multiple RAT nodes, e.g. LTE, WiFi nodes and etc., may coexist with each other. FIG. 1 shows Inter-RAT interference problems such as the well known hidden node problem between LTE-U and LTE-U networks or LTE-UE and WiFi networks. For WiFi to WiFi hidden node problem, the well known CSMA with Collision Avoidance utilizing the RTS/CTS mechanism is used. In such a mechanism, a Listen Before Talk is used where the WiFi AP or terminal would sense for the channel before transmitting any packet.
In order to make LTE-U coexist with other RAT nodes in the unlicensed band, a CSMA utilizing Listen Before Talk mechanism is required. Now for any node in WiFi, it can send a RTS to another node when it wishes to send data to that node, and the recipient node needs to response with a CTS. However, since the uplink in 1st version of LTE-U uses the licensed spectrum, the role of the original CTS is limited. This is because that the neighboring nodes will not listen to the unlicensed band to receive the CTS. Hence it can be appreciated that the Cis feedback in WiFi cannot be used directly in LTE-U.
On the other hand, if a user equipment (UE) reports a discovery reference signal (DRS) based CQI to a serving node to make the serving node determine the situation of the unlicensed channel, a following problem will appear. For LTE-U off-to-on transition, because DRS is periodically transmitted, e.g. 40/80/160 milliseconds, the DRS-based CQI may not fast enough to tell the channel situations. Hence, DRS-based CQI may not be suitable for indicating the channel.
Further, if a UE uses a long term L3 layer report to indicate the situation of an unlicensed channel, a drawback that it could not fast reflect the channel situation will also appear.